Fluxes for use in submerged arc welding are mainly classified into a fused flux and a sintered flux in terms of the form of flux. The fused flux is manufactured by melting various materials in an electric furnace and the like and crushing them. Whereas, the sintered flux is manufactured by bonding various raw materials with a binder, such as alkaline silicate, granulating them, and sintering the granules.
In general, the sintered fluxes are further classified, depending on the sintering temperature, into a low-temperature sintered flux produced by sintering at 400 to 600° C. and a high-temperature sintered flux produced by sintering at 600 to 1,200° C. The low-temperature sintered flux has been conventionally studied from various aspects to reduce the diffusion of hydrogen into a weld metal (see Patent Documents 1 to 3). For example, Patent Documents 1 to 3 disclose a technique in which the ratio of carbonates to the flux is set in a specific range, thereby generating CO2 gas during welding, thus reducing a partial pressure of H2 gas.
To improve moisture absorption properties without using carbonates, another means is proposed to reduce a hydrogen content in the weld metal by specifying an A value, which is a characteristic value mainly derived from a flux component, as well as the maximum value of a specific surface area of the flux (see Patent Document 4). Whereas, in the high-temperature sintered flux, a technique is proposed to decrease the content of diffused hydrogen by specifying the kind of material, such as a basic oxide, an alkali metal fluoride and an acid oxide, as well as a content thereof (see Patent Document 5).